1. Field of the Invention
The present invention relates generally to ophthalmic lenses and, more particularly, to a monofocal ophthalmic lens having an enhanced depth of focus.
2. Description of Related Art
Monofocal ophthalmic lenses provide a single vision correction diopter power. When a prescription does not require any astigmatism correction, and when the prescription can be accurately applied without error, monofocal ophthalmic lenses can provide excellent image quality to the patient. In reality, an exact power prescription for any given patient is often difficult to achieve. Surgeons are facing increasing demand to improve intraocular lens (IOL) power predictions in cataract surgery, for example. Spherical errors within plus or minus 0.5 diopters occur in approximately fifty percent of these cases, and spherical errors within a range of plus or minus 1 diopter occur in approximately ninety percent of these cases.
An inherent variability associated with intraocular lens power calculations results from uncertainties of the parameters of any given ocular system, such as axial length, intraocular lens location, corneal diopter, and intraocular lens tilt and/or decentration. As a result of this inherent variability, the excellent image quality achievable by monofocal intraocular lenses is often obtained at the expense of subsequent, additional refractive correction after the initial implantation of the monofocal intraocular lens.
FIG. 1 illustrates a conventional monofocal ophthalmic lens 10, which has been implanted with a spherical error of approximately 0.5 diopters. This spherical error prevents the parallel incoming light from being focused by the monofocal ophthalmic lens 10 near a retina 12 of the eye of a user. The parallel incoming light comprises peripheral rays 14 and intermediate rays 16, all of which are refracted by the monofocal ophthalmic lens 10 onto the focus point 18. In the illustration of FIG. 1, the spherical refractive error is equal to approximately 0.5 diopters. If the spherical refractive error were greater, light would be focused at a point closer to the monofocal ophthalmic lens 10, between the monofocal ophthalmic lens 10 and the retina 12. If the spherical refractive error were positive, light would be focused behind the retina 12, at a distance proportional to the magnitude of the positive spherical refractive error.
Although the parallel incoming light is refracted only about 0.5 diopters in front of the retina 12, the quality of image perceived by the user is drastically reduced. This quality of image is further reduced as the magnitude of the refractive error increases. Thus, although the monofocal ophthalmic lens 10 is capable of providing excellent image quality correction, the image quality can only be obtained within a relatively narrow depth of focus. The monofocal ophthalmic lens 10 must thus focus the parallel incoming light very close to the retina 12 in order to obtain high image quality. Consequently, the optical quality of the monofocal ophthalmic lens 10 is very sensitive to spherical refractive error.
Turning to FIG. 2, the conventional monofocal ophthalmic lens 20 is shown focusing parallel incoming light onto and near a retina 12. The eye of this illustration comprises a slight cylinder or astigmatism. Although the cylinder generally may comprise curvatures about a number of differently oriented axes, the cylinder of the eye in FIG. 2 comprises a vertical curvature that is different from a horizontal curvature. The solid lines 24 and 26 in FIG. 2 illustrate the refraction of parallel incoming light through a vertical cross section of the monofocal ophthalmic lens 20, and the dashed lines 30 and 32 illustrate the refraction of parallel incoming light through a horizontal cross section of the multifocal ophthalmic lens 20.
The parallel incoming light passing through the vertical cross section of the multifocal ophthalmic lens 20 comprises peripheral rays 24 and intermediate rays 26, all of which are focused along a sagittal line of focus 27. The sagittal line of focus is perpendicular to a plane on which FIG. 2 is drawn. For the purposes of illustration, the peripheral rays 24 and the intermediate rays 26 are shown focused at a point 28 on the sagittal line of focus 27, which is located between the monofocal ophthalmic lens 20 and the retina 22. Since the peripheral rays 24 and the intermediate rays 26 are not focused onto the retina 22, the cylinder of the eye comprises a negative diopter. If the peripheral rays 24 and the intermediate rays 26 were focused by the monofocal ophthalmic lens 20 behind the retina 22, then a positive cylinder diopter error would exist about the vertical axis.
In the illustration of FIG. 2, the peripheral rays 30 and the intermediate rays 32 passing through a horizontal cross section 34 of the monofocal ophthalmic lens 20 are focused onto the tangential line of focus 35. The peripheral rays 30 and the intermediate rays 32 are shown in the figure focused at a point 36 along the tangential line of focus 35. Since the tangential line of focus 35 passes through the retina 22, very little diopter error exists along the horizontal cross section 34 of the eye. The peripheral rays 30 and the intermediate rays 32 would focus in front of the retina 22 in the case of a positive diopter error along the horizontal cross section 34, and would focus behind the retina 22 in the case of a negative diopter error along the horizontal cross section 34.
The distance between the sagittal line of focus 27 and the tangential line of focus 35 is referred to as the cylinder power. This astigmatism may be corrected by adding cylinder power and spherical power. More particularly, cylinder power, which is opposite to the existing cylinders is added, to thereby collapse the sagittal line of focus 27 and the tangential line of focus 35 into a single point located between the sagittal line of focus 27 and the tangential line of focus 35. Spherical power is then added to bring this single point onto the retina 22. Similarly to the inherent variabilities discussed above with reference to FIG. 1, precise astigmatism correction is often difficult to achieve. In the case of an intraocular lens having a torric surface for astigmatism correction of the cornea, the use of the torric surface with the regular spherical intraocular lens suffers from a number of inherent problems.
An object of a conventional torric intraocular lens, for example, is to reduce the preexisting cylinder and, as a result, to reduce spectacle dependency. The torric intraocular lens, however, may increase the spherical error of the system and still require the subject to wear glasses. As an example, if a minus 3 diopter cylinder is used to correct a plus 3 corneal cylinder and the torric intraocular lens is rotationally misaligned by 30 degrees, then the ocular system will suffer an approximately negative 0.5 diopter spherical shift from the targeted condition of the perfect alignment. This spherical shift is clinically significant, and even lesser spherical errors may be uncomfortable to the patient. A need has thus existed in the prior art of both monofocal and torric ophthalmic lenses for a device capable of reducing the sensitivity to refractive errors of these lenses.